Aging Pioneer 6 Spacecraft Dips Closer to Earth

The elder statesman among orbiting spacecraft will jog past Earth on Saturday, passing closer than it has been since it was launched 23 years ago, and the maneuver will forever change its course.

It won’t be all that close--1.16 million miles--but that is close enough to set the venerable craft on a course that will lengthen its year by six days. From that point on, it will take 317 days for Pioneer 6 to orbit the sun.

The change in course will result from the Earth’s gravitational pull. Although the 140-pound spacecraft will come no closer than five times the distance to the moon, the Earth’s pull will be strong enough to tug the craft closer to the planet’s own orbit, which is farther out from the sun than Pioneer 6 has been traveling. That, in turn, will add 6 million miles to the satellite’s orbit around the sun.

Pioneer 6 is one of those durable little mechanical birds built in Redondo Beach by TRW Space & Technology Group that just seem to go on forever. Long past its life expectancy, Pioneer 6--along with three sister probes operated by NASA’s Ames Research Center in Mountain View--has sent back enough information on the sun to force scientists to change many of their earlier conclusions.

The satellites were designed to study the sun’s atmosphere, called the heliosphere, and the solar wind. Before the satellites began their work, scientists had thought that the solar wind was a gentle, steady flow.

But the satellites revealed that the surface of the sun is intensely violent, and the solar wind boils off into interplanetary space with such fury that it influences events in the outer reaches of the solar system.

The solar wind is what causes comets to have tails that can be seen from Earth, and it consists mainly of subatomic particles traveling at about 1 million miles an hour.

Pioneer 6 also has measured the outer surface of the sun, called a corona. Radio signals from the small craft must pass through the corona when Pioneer circles behind the sun, and changes in the signals have told scientists much about the nature of the corona.

Only two of Pioneer’s six original instruments are still working, and they continuously send back data on cosmic rays, gases and particles that flow from the sun. Unfortunately, much of the time that the craft is sending back data, no one is listening these days.

The solar-powered spacecraft can only be monitored by NASA’s Deep Space Network, a series of antennas around the globe that is in demand for many space projects. Other pressures for the network are so great that Pioneer’s continuous flow of information has to be ignored most of the time.

During its extraordinary lifetime, Pioneer 6 has transmitted 10 billion bits of information back to Earth, and it has traveled 14 billion miles, according to the Ames Research Center. It passes by the Earth once every 5 3/4 years.

This time, however, it will come just a tad closer.

But even though it will be a little closer, the small craft, which has more than 56,000 parts and measures only 35 inches tall, will be too faint to be seen from Earth.